The Pediatric HIV/AIDS Cohort Study (PHACS) was created in 2005 to evaluate the clinical course of perinatally acquired HIV infection among adolescents and pre-adolescents and the consequences of fetal and neonatal exposure to HIV and antiretroviral chemotherapy among a representative cohort of children in the United States. A cohort of 450 perinatally infected adolescents and preadolescents (Adolescent Master Protocol, AMP, age 7-16 at enrollment) was established to evaluate the impact of HIV and ART on sexual maturation, pubertal development, and socialization; and, a drug toxicity surveillance system (Surveillance Monitoring for Anti-Retroviral Toxicities Study (SMARTT) enrolled 1,934 perinatally HIV exposed uninfected children to evaluate long-term effects of in-utero ART exposure. PHACS is comprised of a Scientific Leadership Group (SLG), which is overseen by a Coordinating Center, a Data and Operations Center (DOC), and 24 clinical sites. The Department of Epidemiology and the Center for Biostatistics in AIDS Research (CBAR) at the Harvard School of Public Health, Westat, and the Frontier Science Foundation collaborate to form the PHACS DOC. The DOC collaborates with the SLG to define the PHACS research agenda; provides methodological support for the development of all PHACS analytic projects; merges data from pre-existing databases from previous cohorts (PACTG 219/219C, WITS, Legacy); maintains clinical site subcontracts and trains and monitors sites in proper procedures for PHACS research; plans and conducts all leadership and full PHACS network meetings; and, supports an active CAB. In PHACS II, the DOC will continue the duties described above while refining its practices, as well as follow and enroll an additional 1,200-1,500 children into SMARTT. Together, HSPH, Westat and Frontier Science bring long histories of providing the type of methodologic and operational support required by PHACS, as well as innovative methods to enhance and maximize the efficiency of PHACS study design, conduct, and analysis. Given our prior and current professional experience, we are uniquely positioned to provide the scientific/epidemiologic and operational leadership to successfully conduct PHACS.